I am working on an a Raspberry Pi based, Internet connected indoor air quality monitor called LiV Pi. LiV is an open source hardware and software project. LiV measures temperature, humidity, air pressure and CO2 levels. In order to keep the price low, Raspberry Pi does not come with a Real Time Clock, like a regular computer or laptop, but uses instead NTP protocol to retrieve the correct date at boot time. For this to work, the RPi has to be connected to Internet when it boots, so it can reach a NTP server and get the correct time. When you power up your Raspberry Pi without a network connection, the date set on the Pi is the timestamp that was saved in its memory when the Pi was turned off last time. This is obviously not accurate. If you want accurate time stamps for your measurements when your Raspberry Pi boots without being connected to the network, then you need a Real Time Clock. For my LiV project, I am using a modified Arduino DS3231 module (pull-up resistors to 5V removed).